Process for recovering lower carbon olefins from product gas for production of olefins

ABSTRACT

The present invention concerns a process for recovering lower carbon olefins from MTO or DTO product gas. Said process primarily comprises the product gas compressing, pre-deethanizing, demethanizing and ethylene recovering apparatus, depropanizing column, ethylene rectification column, propylene rectification column and the like. In addition, the process of the present invention needs no independent ethylene cooling system, and the ethylene recovery rate may achieve 99.5%.

TECHNICAL FIELD

The present invention relates to a process for recovering low carbon olefins from a product gas obtained from an olefin conversion device. More specifically, the present invention concerns a process for recovering ethylene and propylene from the product gas from the methanol to olefin device and/or dimethyl ether to olefin device.

BACKGROUND ART

It is learned that the demand for low carbon olefins is increasing, but the starting material resources, e.g. naphtha, light diesel oil and so on, for the production of ethylene is confronted with serious shortage. The methanol to olefin (hereinafter referred to as MTO) or dimethyl ether to olefin (hereinafter referred to as DTO) technique, therefore, has drawn much attention. MTO or DTO technique is a technology for producing low carbon olefins, such as ethylene, propylene and the like, by using coal-based or natural gas-based synthesized methanol or dimethyl ether as the starting materials and by means of a fluidized bed reaction form similar to a catalytic cracking device. Said technology can be used for the production of low carbon olefins with a high selectivity, and the propylene/ethylene ratio may be optionally adjusted within a relatively broad scope.

Generally, the product gas obtained by the catalytic reaction of methanol via a MTO reaction device contains hydrogen, methane, ethylene, ethane, propylene, propane, and high carbon olefins, such as butylene or pentene, as well as water, carbon monoxide and carbon dioxide. The reaction products of the cracking of DTO are similar to those of MTO, but the conditions for DTO production are readily controllable. In order to obtain ethylene products in a polymerizable grade, the separation and recovering techniques of MTO product gases are very essential. Generally, the technique for recovering ethylene comprises, before recovering olefins, subjecting the MTO product gas to the conventional gas-liquid separation step to separate and remove water, carbon dioxide and heavy components of C₅ and higher; cooling and introducing C₄ & light components into the deethanizing column; discharging ethane and light components from the top of the deethanizing column; and then feeding them into the demethanizing column after removing ethyne via a hydrogenation device; removing methane and hydrogen from the top of the demethanizing column, and introducing the materials at the column bottom substantially containing ethylene and ethane into the C₂ component separation column; and obtaining ethylene products at the top of the C₂ component separation column. U.S. Pat. No. 5,811,621 has detailedly described the process. In said technology for recovering ethylene, many compression and pressure-increasing processes are required. Generally, the pressure of C₄ & light components is required to be increased to 20-30 atm.A before feeding C₄& light components into the deethanizing column. In addition, another pressure-increasing step is further needed to increase the pressure to higher than 30 atm.A after removing ethyne via the hydrogenation reaction device and before feeding C₄& light components into the demethanizing column. In order to increase the recovery rate of ethylene, the materials at the top of the demethanizing column are further compressed and pressure-increased, and the compressed gas provides heat for the reboiler of the demethanizing column, and then are partially condensed. The uncondensed gas contains ethylene in an amount of 3-4% of the feedstock, and is discharged. It can be seen that three processing compressors and a propylene refrigeration compressor are required in said ethylene recovering technology, which increases the equipment investment. Meanwhile, said uncondensed gas is discharged in said technology so as to lose a small amount of ethylene. Accordingly, the ethylene recovery rate is not satisfactory, and generally about 95-97%. Accordingly, it highly needs a process for recovering low carbon olefins from a product gas from the methanol to olefin device and/or dimethyl ether to olefin device. Said process not only lowers energy consumption, but also may be used for recovering low carbon olefins in a high yield, so as to obtain ethylene and propylene products in the polymerizable grade.

SUMMARY OF THE INVENTION

The present invention provides a process for recovering low carbon olefins from MTO and/or DTO product gases, comprising the operation units of product gas pressure increasing, separation of C₂ & light components and C₃ & heavy components, demethanization of C₂ & light components, rectification of ethylene and rectification of propylene, wherein said process is characterized in setting up an ethylene recovering device in the demethanizing unit of C₂ & light components so as to recover ethylene in the uncondensed gas, and using the liquid phase C₂ components in the separation unit of C₂ & light components and C₃ & heavy components and/or the ethylene rectification unit as a coolant in said ethylene recovering device.

Since DTO flow scheme is substantially the same as MTO flow scheme, only MTO reaction device is detailedly illustrated as follows.

Generally, 80% or more (based on carbon element) methanol is industrially converted to ethylene and propylene after the catalytic reaction of methanol in MTO reaction device. Meanwhile, a small amount of methane, carbon dioxide, oxide and hydrocarbons of C₄ or higher are also produced, in addition to an extremely small amount of hydrogen and ethyne. In most cases, most oxides are removed from the gas from the MTO reactor after rapid cooling and washing, and the pressure thereof is less than 0.2 MpaG. Such materials are the MTO product gas as called in the present invention. The process for recovering low carbon olefins of the present invention can be used to recover low carbon olefins, i.e. mainly ethylene and propylene, from such MTO product gas, and both ethylene and propylene obtained thereby can reach the polymerization grade standard.

Preferably, the process for recovering low carbon olefins of the present invention comprises the steps of:

-   -   (1) increasing the product gas pressure to 2.0-4.0 MpaG;     -   (2) cooling, drying and introducing the pressure increased MTO         product gas into the deethanizing column 10, to separate C₂ &         light components from C₃ & heavy components;     -   (3) hydrogenating the materials at the top of the deethanizing         column 10 and feeding into the demethanizing column 13, to         remove a small amount of methane and hydrogen;     -   (4) feeding the materials at the bottom of the demethanizing         column 13 into the ethylene rectification column 22, to obtain a         qualified ethylene product; and     -   (5) feeding the materials at the bottom of the deethanizing         column 10 into the depropanizing column 23, to separate out C₃         components, and then feeding C₃ components into the propylene         rectification column 25 to obtain a qualified propylene product;     -   wherein the materials at the top of the demethanizing column 13         are cooled and fed into a reflux tank 17; the tail gas in the         reflux tank 17 is fed into an ethylene recovering device         comprising at least a condenser 19, wherein the coolant in the         condenser 19 is one selected from the group consisting of         -   a. C₂ components at the top of the deethanizing column 10             and/or the reflux tank thereof; and         -   b. C₂ mixture and/or ethylene in the ethylene rectification             column system.

Specifically, the present invention provides a process for recovering low carbon olefins from MTO product gases, mainly comprising a pressure-increasing and acidic gas removing system, a C₂ & light component and C₃ & heavy component separation system, an ethylene rectification system and a propylene rectification system. Further explanations are as follows by reference to FIG. 1. The process for recovering low carbon olefins of the present invention comprises:

(1) Pressure-increasing and acidic gas removing system. In said system, the pressure of the product gas 1 from the MTO device is increased via a pressure-increasing apparatus, e.g. processing gas compressor 2, to 2.0-4.0 MpaG, preferably 2.0-3.0 MpaG, and an oxide removing system 5 is set up at a suitable site (e.g. between sections 3 and 4 of the compressor). Unreacted methanol in the feedstock and dimethyl ether produced in the reaction are removed via washing and stripping, and simultaneously an acidic gas removing system 6 is set up to remove CO₂ produced in the reaction via alkaline washing.

(2) C₂ & light component and C₃ & heavy component separation system. In said system, the gas obtained from said pressure-increasing and acidic gas removing system is cooled and fed into a desiccator to remove water. An adsorbing system, if necessary, may also be set up to remove some other impurities, such as oxides. Then, the gas is fed into the deethanizing column 10 so as to separate C₂ & light components from C₃ & heavy components, wherein the operation temperature of said deethanizing column 10 is generally from 0 to −25° C. The materials at the top of the deethanizing column 10 are introduced into the C₂ hydrogenating system 12 to remove a trace amount of ethyne from the materials, so as to satisfy the specification requirements of the ethylene products. The C₂ hydrogenating system 12 is usually a fixed-bed reactor, and G58C from Sud Chemie or other common hydrogenation catalysts in the field may be used. Additionally, a small amount of hydrogen may also be supplemented if the amount of hydrogen in the MTO product gas cannot satisfy the hydrogenation requirements.

(3) Demethanizing system. The hydrogenated product gas is cooled and fed into the demethanizing column 13, wherein operation temperature of the demethanizing column 13 is generally from −30 to −37° C. Said demethanizing column is used to make the ethylene product satisfy the quality requirements, but it is more important that it is used to recover ethylene from the product gas. The materials at the top of the demethanizing column 13 are cooled and fed into the reflux tank 17. Due to the presence of methane and hydrogen, the materials at the top of the demethanizing column 13 are not completely condensed. There is accordingly uncondensed tail gas in the reflux tank 17, and said tail gas contains a small amount of ethylene. If said tail gas is discharged or burned as fuel gas, there will be much economic loss. Therefore, in accordance with the present invention, an ethylene recovering device is set up on the reflux tank of the demethanizing column 13.

Said ethylene recovering device comprises at least a condenser 19, which is generally a flat type one.

In order to achieve better separation effects, one preferred embodiment involves that said ethylene recovering device further comprises a rectification column section 18 directly linking with the demethanizing column reflux tank 17. The uncondensed tail gas in the reflux tank 17 is fed into the. rectification column section 18. Various internal column components may be used inside the rectification column section 18, e.g. fillings, float valves and the like. Generally, the operation temperature of said rectification column section 18 ranges from −37 to −98° C. Said plate type condenser 19 is set up at the top of the rectification column section 18. The ethylene recovering device consisting of the rectification column section 18 and the condenser 19 is used to further recover ethylene in the uncondensed gas by using the fractionation principle, so as to increase the ethylene recovery rate.

In another embodiment of the process of the present invention, the condenser 15 at the top of the demethanizing column 13, the reflux tank 17, the rectification column section 18 and the plate type condenser 19 at the top of the rectification column section 18 are combined together and set up at the top of the demethanizing column 13 (see FIG. 2). Such a compact structure needs no reflux pump, but C₂ component coolant needs sufficient pressure to reach the top flat type condenser 19.

Coolant at a temperature of less than −40° C. is usually needed for the condenser 19 in the ethylene recovering device in any form above. In order to avoid designing any new refrigeration system, e.g. ethylene refrigeration system, and lower energy consumption, C₂ components produced by the system per se are sufficiently utilized in the present invention to provide the refrigeration as required. The coolant in the condenser 19 is one selected from the group consisting of

-   -   a. C₂ components at the top of the deethanizing column 10 and/or         the reflux tank thereof; and     -   b. C₂ mixture and/or ethylene in the ethylene rectification         column system.

After the pressure of the coolant is reduced via the throttle valve 14, it is fed into the condenser 19 of the ethylene recovering device. In said condenser 19, said coolant is vaporized so as to provide the refrigeration for the ethylene recovering device, and then refluxed back to suitable site of the compressor 2 (determined by pressure) after the refrigeration is further recovered by the system.

FIG. 1 and FIG. 2 illustrate the flow scheme of drawing out C₂ component coolant from the deethanizing column.

(4) Ethylene rectification system. The materials at the bottom of the demethanizing column 13 are fed into the ethylene rectification system 22. At the top of the ethylene rectification system 22 is obtained the ethylene product in a polymerizable grade, which has an ethylene content of higher than 99.9 mol %. Ethane obtained at the bottom of the ethylene rectification column 22 can be delivered out as a product or incorporated into the fuel system. Since there are only C₂ components in the materials fed into the ethylene rectification column 22, open-type heat pump technique may be used in the ethylene rectification column, if required by the consumer, to reduce the energy consumption.

(5) Propylene rectification system. The materials at the bottom of the deethanizing column 10 are fed into the depropanizing column 23. C₃ components are separated out at the top of the depropanizing column 23, and C₄ components and higher are separated out at the bottom of the depropanizing column 23. C₃ components at the top of the ethanizing column 23 are fed into the propylene rectification column 25. At the top of the propylene rectification column 25 is obtained the propylene in a polymerizable grade, which has a propylene content of higher than 99.5 mol %. Propane obtained at the bottom of the propylene rectification column 25 can be delivered out as a product or incorporated into the fuel system.

In a specific embodiment, if the common engineering system is deficit in steam, the system of the combination of the rectification column with the open-type heat pump technique may be used as the propylene rectification system to reduce the energy consumption of the device. Meanwhile, the combination with propylene refrigeration compressor may also be used to reduce the compressor units. Specifically, the gas at the top of the propylene rectification column 25 is firstly fed into the heat pump compressor 27 so as to increase the pressure to a sufficient extent and to condense propylene in the reboiler 28. A part of condensed propylene is taken out, and the remaining is refluxed back to the top of the column. Therefore, the propylene rectification column needs no steam to provide heat for the reboiler.

Under the circumstance of the presence of sufficient steam, particularly waste steam, a common flow scheme may be used in the propylene rectification system.

The process for recovering low carbon olefins of the present invention should also comprise a conventional closed circuit refrigeration system using propylene as a coolant to provide the refrigeration of various temperature ranges for the separation parts.

It is noteworthy that the process for recovering low carbon olefins from the MTO product gas of the present invention have notably advantages over the prior art:

-   -   1. feeding part or all of the tail gas in said demethanizing         column reflux tank into the ethylene recovering device, to         further recover ethylene in the uncondensed gas by using the         fractionation principle and to increase the ethylene recovery         rate, which may be higher than 99.5%; and     -   2. after throttling and pressure-reducing, using the liquid         materials from the deethanizing column reflux tank or ethylene         rectification column as the coolant of said ethylene recovering         device; and vaporized coolant returns to a suitable site of the         processing gas compressor without any separate ethylene         refrigeration compressor, so as to save energy and cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the flow scheme of the process for recovering low carbon olefins from the MTO product gas, wherein the ethylene recovering device comprises the rectification column section 18 and the plate type condenser 19 at the top thereof.

-   1—MTO product gas; -   2—Product gas compressor; -   3—Pressure-increased product gas; -   4—Condenser at the outlet of the compressor; -   5—Oxide removing system; -   6—Acidic gas removing system; -   8—Product gas at the outlet of the compressor; -   9—Condenser at the outlet of the compressor; -   10—Deethanizing column; -   11—C₂ component coolant; -   12—Ethyne hydrogenation; -   13—Demethanizing column; -   14—C₂ component coolant throttle valve; -   15—Condenser at the top of the demethanizing column; -   16—Demethanizing column reflux; -   17—Demethanizing column reflux tank; -   18—Rectification section at the top of the demethanizing column     reflux tank; -   19—Plate type condenser; -   20—Methane tail gas; -   21—Feedstock of ethylene rectification column; -   22—Ethylene rectification column; -   23—Depropanizing column; -   24—Feedstock of propylene rectification column; -   25—Propylene rectification column -   26—C₂ coolant return; -   27—Heat pump compressor; -   28—Ethylene rectification column reboiler; -   29—Ethane product; -   30—Propylene product; -   31—Products of C₄ or higher -   32—Ethylene product; -   33—Propane product.

FIG. 2 shows the flow scheme of the process for recovering low carbon olefins from the MTO product gas, wherein the condenser 15 at the top of the demethanizing column 13, the reflux tank 17, the rectification column section 18 and the plate type condenser 19 at the top thereof are combined together and set up at the top of the demethanizing column 13.

EMBODIMENTS

The present invention is detailedly disclosed by the following examples, but it is not limited by said examples.

EXAMPLES

According to the scheme as shown in FIG. 1, low carbon olefins were recovered from the MTO product gas. The composition (mol %) of the MTO product gas fed into the separation system was shown in Table 1, wherein the temperature thereof was 41° C., and the pressure was 135 kPaA.

TABLE 1 Composition of the Feedstock Component CO Methane ethylene ethane propylene propane C₄ C₅ and higher Content 0.23 2.30 61.06 4.61 24.42 0.92 5.53 0.93 (mol %)

The scheme simulation for the scheme of FIG. 1 was conducted by using the general PROVISION 6.0 scheme simulation software from Simsci, to obtain the simulation computation data results as shown in Table 2.

The computation results showed that the recovery rate of ethylene may achieve 99.5% by using the process of the present invention to separate the MTO product gas.

TABLE 2 Discharged Gas at the Fluid at the Fluid at the Tail gas in the MTO at the top of the bottom of the bottom of the demethanizing product end of the deethanizing deethanizing demethanizing column Ethylene Recycled Material stream gas compressor column column column reflux tank product C₂ Phase state Vapor Vapor Vapor Liquid Liquid Vapor Liquid Vapor Temperature, ° C. 41.000 83.110 −17.138 75.405 −17.451 −84.307 −33.036 15.000 Pressure, KPA 135.000 2850.000 2755.000 2805.000 2650.000 2550.000 1800.000 177.000 Average molecular weight 33.383 33.024 27.798 45.384 28.201 18.241 28.054 28.153 Total flow, KG-MOL/HR 4340.000 4660.003 2961.614 1378.417 2841.593 120.011 2647.313 319.973 Mole fraction % H2 CO 0.0023 0.0022 0.0034 0.0000 0.0000 0.0833 0.0000 0.0005 CO₂ METHANE 0.0230 0.0222 0.0338 0.0000 0.0007 0.8167 0.0001 0.0109 ACETYLN ETHYLENE 0.6106 0.6301 0.8948 0.0000 0.9283 0.1000 0.9995 0.8953 ETHANE 0.0461 0.0491 0.0671 0.0010 0.0699 0.0000 0.0004 0.0898 PROPENE 0.2442 0.2277 0.0010 0.7669 0.0010 0.0000 0.0000 0.0035 PROPANE 0.0092 0.0086 0.0000 0.0290 0.0000 0.0000 1BUTENE 0.0553 0.0515 0.0000 0.1741 0.0000 0.0000 1PENTENE 0.0092 0.0086 0.0000 0.0290 0.0000 

1. A process for recovering lower carbon olefins, from a product gas, wherein said product gas is a mixed gas obtainable from the methanol and/or dimethyl ether to olefin conversion, said process comprising deethanizing said product gas to obtain C₂ and light components at the top of the column and C₃ and heavy components at the bottom of the column, and demethanizing C₂ and light components to obtain a tail gas and liquid phase C₂ component, condensing said tail gas and recovering the condensed liquid phase back to said demethanizing step.
 2. The process according to claim 1, further comprising increasing the product gas pressure to 2.0-4.0 MpaG before separation of the product gas.
 3. The process according to claim 1, wherein the liquid phase material extracted from the top of the deethanizing column is used as a coolant for condensing the tail gas at a temperture from −60 to −98° C.
 4. The process according to claim 1, further comprising ethylene rectifying the liquid phase C₂ component to obtain an ethylene product and ethane.
 5. The process according to claim 4, wherein said ethylene product or ethane is replaceably or additionally used as a coolant for condensing the tail gas.
 6. The process according to claim1, further comprising depropanizing C₃ and heavy components, separating out C₃ component, and propylene rectifying said C₃ component to obtain a propylene product.
 7. The process according to claim 1, further comprising rectifying the tail gas before condensation to recover ethylene in said tail gas, and recovering said ethylene back to said deethanizing step.
 8. The process according to claim 1, wherein the tail gas is condensed with a condenser.
 9. The process according to claim 8, wherein the tail gas is condensed with a plate type condenser or a plate-fin type condenser.
 10. The process according to claim 1, wherein the product gas is a mixed gas obtained in the methanol to olefin process.
 11. The process according to claim 10, comprising (1) increasing the product gas pressure to 2.0-4.0 MpaG; (2) introducing the pressure increased product gas into the deethanizing column (10), to separate C₂ and light components from C₃ and heavy components; (3) feeding C₂ and light components into the demethanizing coulm (13), to obtain the tail gas and liquid phase C₂ component; (4) feeding the liquid phase C₂ component into the ethylene rectifying column (22), to obtain an ethylene product and ethane; and (5) feeding C₃ and heavy components into the depropanizing column (23), to separate out C₃ component, and feeding C₃ component into the propylene rectifying column (25), to obtain a propylene product; wherein the materials at the top of the demethanizing column (13) are fed into the reflux tank (17) to separate the tail gas from the mixed C₂ component stream; the tail gas in the reflux tank (17) is fed into the condenser (19) for condensation, and the condensed material stream is refluxed back to the demethanizing column (13), wherein the coolant in the condenser (19) is one selected from the group consisting of c. the material stream extracted from the top of the deethanizing column, or mixed C₂ component stream extracted from the reflux tank; and d. ethylene product and/or ethane.
 12. The process according to claim 11, comprising rectifying the tail gas via the rectification column section (18) before condensing the tail gas, wherein the rectification column section (18) is linked with the reflux tank (17) at the bottom of said section, and with the condenser (19) at the top of said section.
 13. The process according to claim 12, wherein the condenser (19( is a plate type condenser or a plate-fin type condenser.
 14. The process according to claim 13, wherein the reflux tank (17), the rectification column section (18), and the plate type condenser are integrated together.
 15. The process according claim 11, wherein the demethanizing column (13) further comprises the column top condenser (15), and said column top condenser (15), the reflux tank (17), the rectification column section (18), and the plate type condenser are integrated together.
 16. The process according to claim 10, comprising (1) increasing the pressure of the product gas (1) to 2.0-4.0 MpaG via the pressure-increasing device (2), and removing unreacted methanol and dimethyl ether and CO₂ produced thereby in the product gas (1); (2) subjecting the pressure-increased product gas to gas liquid separation after colling, feeding the gas phase into a desiccator to remove water, and then into the deethanizing column (10) to separate C₂ and light components from C₃ and heavy components, feeding C₂ and light components into the C₂ hydrogenation system (12) to remove ethylene, to obtain hydrogenated material stream; (3) feeding the hydrogenated material stream into the demethanizing column (13) after cooling, the materials at the top of the demethanizing column (13) being fed into the reflux tank (17) to separate the tail gas from the mixed C₂ component stream, the tail gas in the reflux tank (17) being fed inti the rectification column section (18) having the condenser (19) at the top thereof, the condensed material stream being refluxed back to the demethanizing column (13), wherein the coolant in the condenser (19) is one selected from the group consisting of a. the material stream extracted from the top of the deethanizing column, or the mixed C₂ component stream extracted from the reflux tank; b. ethylene product and/or ethane. (4) the materials at the bottom of the demethanzing column (13) being fed into the ethylene rectification column (22) to obtain ethylene product at the top the column (22) and ethane at the bottom the column (22); and (5) the materials at the bottom of the deethanizing column (10) are fed into the depropanizing column (23) to obtain C₃ components at the top of the column (23) and ethane at the bottom the column (23); and then C₃ components are fed into the propylene rectification column (25) to obtain propylene product at the top of the column (25) and propane at the bottom the column (25). 